A display device utilizing a liquid crystal compound (in this patent application, the term, the liquid crystal compound, is used as a generic term for a compound that exhibits a liquid crystal phase such as a nematic phase or a smectic phase, and a compound that exhibits no liquid crystal phases but useful as a component of a liquid crystal composition) has been widely used for the display of a watch, a calculator, a word processor or the like. The display device utilizes the refractive index anisotropy, the dielectric anisotropy and so forth of the liquid crystal compound.
A liquid crystal phase includes a nematic liquid crystal phase, a smectic liquid crystal phase, a cholestric liquid crystal phase, and the nematic phase is most widely applied. A display mode includes a DS (dynamic scattering) mode, a DAP (deformation of aligned phases) mode, a GH (guest-host) mode, a TN (twisted nematic) mode, a STN (super twisted nematic) mode, a TFT (thin film transistor) mode, a VA (vertical alignment) mode, an IPS (in-plane switching) mode and a PSA (polymer sustained alignment) mode.
A liquid crystal compound used for these display modes is required to exhibit a liquid crystal phase in a wide temperature range, centering at room temperature, to be sufficiently stable under conditions that the display device is used, and also to have sufficient characteristics for driving the display device. However, no single liquid crystal compounds that satisfy these conditions have been found until now.
The actual situation is that a liquid crystal composition is prepared by mixing from several to several tens of liquid crystal compounds in order to satisfy the required characteristics. It is required that the liquid crystal composition is stable to moisture, light, heat and air, which are normally present under conditions that the display is used, and is stable to an electric field or electromagnetic radiation, and is also stable chemically to a compound that will be mixed. It is required that the liquid crystal composition has suitable values of a variety of physical properties such as refractive index anisotropy (Δn) and dielectric anisotropy (Δ∈), depending on the display mode or the shape of the display device. Furthermore, it is important that each component in the liquid crystal composition has an excellent solubility in each other.
It is desirable for an excellent liquid crystal display that the cell thickness of the liquid crystal display device used and the value of Δn of the liquid crystal material used are constant. See E. Jakeman, et al., Phys. Lett., 39A. 69 (1972). The response speed of the liquid crystal display device is inversely proportional to the square of the cell thickness. Accordingly, a liquid crystal composition having the value of a large Δn should be used in order to produce a liquid crystal display device that is able to respond at high speed and thus can be applied to the display of moving images and so forth. A variety of compounds as a component having the value of a large Δn for liquid crystals have been synthesized until now. Since such a compound having a large Δn generally has a highly conjugated molecular structure, the compound has a tendency to have a poor compatibility with the other liquid crystal materials, and thus it is not easy to use the compound as a component of a liquid crystal composition having excellent electrical characteristics. Further, a high stability is required in a liquid crystal compound used as a component of a liquid crystal composition, which is required to have a high insulation (specific resistance), for use in a liquid crystal display device having a thin film transistor mode.
In the operating mode described above, the IPS mode, the VA mode, the PSA mode or the like utilizes homeotropic orientation of liquid crystal molecules, and it is known that a limited viewing angle, which is a disadvantage of a conventional display mode such as the TN mode and the STN mode, can be improved by means of these modes.
A variety of liquid crystal compounds, where hydrogen on the benzene ring had been replaced by fluorine, has conventionally been studied as a component of a liquid crystal composition having negative dielectric anisotropy, which is usable for liquid crystal display devices having these operating modes. See the patent documents No. 1 to No. 4.
For example, the compound shown by formula (s-1), where hydrogen on the benzene ring had been replaced by fluorine, has been studied in the patent document No. 1. A compound having alkenyl shown by formula (s-2), where hydrogen on the benzene ring had been replaced by fluorine, has been studied in the patent document No. 2.

However, the compound shown by formula (s-1), where hydrogen on the benzene ring has been replaced by fluorine, has a small optical anisotropy, and the optical anisotropy is not sufficiently large even in the compound shown by formula (s-2). The patent document No. 3 discloses the compounds shown by formulas (s-3), (s-4) and (s-5) as a compound where the lateral group is a polar group such as halogen.
In addition to these, the patent document No. 4 discloses the compound shown by formulas (s-6), (s-7), (s-8) and (s-9), where the lateral group is a polar group such as halogen, however a compound having a biphenyl ring with a fluorine atom or a chlorine atom in the 2-, 3- and 3′-positions and a —CH2O— and —COO— bonding group, as this patent application shows, is not disclosed.

Further, all of the compounds shown by formulas (s-3), (s-4) and (s-5) have a small optical anisotropy, a low maximum temperature of a nematic phase and no liquid crystal phases. Furthermore, since all of the compounds shown by formulas (s-5), (s-7), (s-8) and (s-9) have a small optical anisotropy, a low maximum temperature of a nematic phase and a small dielectric anisotropy, they cannot decrease the driving voltage in a liquid crystal composition including them. The compounds shown by formulas (s-3) and (s-6) do not have a sufficient compatibility at low temperature, for instance, and a further improvement remains to be done.